Conventionally, an urethane coated film formed from a polyurethane coating has extremely excellent flexibility, chemical resistance and stain resistance, and besides, a coated film particularly using a non-yellowing polyisocyanate derived from hexamethylene diisocyanate (hereafter referred to as HDI) as a curing agent further has excellent weather resistance and accordingly increases its demand.
Since interest in protection of the global environment has been recently increased, a technology of decreasing the viscosity of a polyisocyanate compound used as a curing agent has been actively developed. This is because an amount of an organic solvent used in a coating composition can be reduced by decreasing the viscosity of the polyisocyanate compound. (Patent Document 1)
On the other hand, the technologies of decreasing the viscosity have been disclosed, which include various polyisocyanates derived from HDI. One technology among them relates to polyisocyanate having an urethodione group, which has low viscosity (Patent Document 2). The technology provides a polyisocyanate compound having low viscosity, but the polyisocyanate compound has low crosslinkability, and occasionally increases the concentration of a diisocyanate monomer while being stored, so that the use of it is limited.
In polyisocyanate containing only the urethodione group, the statistical average number of isocyanate groups contained in one polyisocyanate molecule (hereafter referred to as average number of isocyanate groups) is 2 regardless of the molecular weight, and the crosslinkability is also inferior.
It has been difficult to simultaneously decrease the viscosity and increase the crosslinkability of polyisocyanate having the urethodione group, because polyisocyanate having an isocyanurate group is also by-produced when the polyisocyanate having the urethodione group is produced. Such polyisocyanate having the urethodione group has been desired as to keep the crosslinkability, have excellent storage stability and show lower viscosity.
Patent Document 1: JP-A-05-222007
Patent Document 2: Japanese Patent No. 3055197